This invention relates to a liquid pressure source unit which is to be mounted on a brake system of a vehicle, etc., and particularly to a liquid pressure source unit of the type which includes a throttle valve unit for damping pulsation pressure and operated to supply power pressure to a liquid-operated booster.
In general, a liquid pressure source unit of this type comprises a hydraulic pressure pump actuated by an external power and adapted to draw hydraulic liquid and discharge the same, and an accumulator arranged in such a manner as to accumulate the pressurized liquid discharged from the pump in a predetermined range of pressure and supplies such accumulated pressurized liquid to a liquid-operated brake circuit. In this liquid pressure source unit, the pressurized liquid discharged from the hydraulic pressure pump is liable to generate pulsation, which gives rise to the problem of vibrations and noises. This problem is more significant in a reciprocal type pump for reciprocating a plunger using an eccentric cam than in a radial piston type pump in which a piston disposed within a rotor is reciprocally moved in a radial direction of the rotor by rotating the rotor relative to an eccentric stator. The pulsation pressure also affects adversely to the accumulator for accumulating the pressurized liquid discharged from the pump and even to a liquid-operated booster which utilizes the pressurized liquid of the accumulator, thereby reducing their tolerance. The pulsation is even further transmitted to a brake pedal in the brake circuit to adversely affect the driver's feel of operation with respect to the pedal.
In view of the above, Japanese Laid-Open Utility Model Application No. Hei 3-129785 discloses a technique for reducing vibrations and noises due to pulsation pressure by providing a pulsation damping device which includes a pressure receiving member which under the influence of the pressure discharged by a reciprocal type pump is moved toward an outlet port of the pump against the force of a spring. The technique for providing an elastic pressure receiving member at an outlet port of a pump is also known from U.S. Pat. No. 5,096,400. Another U.S. Pat. No. 4,988,147 discloses a technique for providing an accumulation chamber and a fixed throttle having a constant throttling function at an outlet port of a pump. The diameter of the fixed throttle is, for example, 1 mm or less. Taking the load of the pump into consideration, the diameter of the fixed throttle is 0.1 mm order (normally 00.5 mm, for example). Incidentally, the liquid-operated booster itself is well known from, for example, U.S. Pat. No. 5,168,791 and some others. Those techniques for reducing such pulsation pressure are good enough for reducing vibrations and noises of the system including the hydraulic pressure pump, the accumulator, and the liquid-operated booster, and are also effective in enhancing the tolerance of the devices.
However, a new problem of a foreign sound has been closed up at a stage where the pulsation pressure has been reduced. This foreign sound is liable to occur to the inside of the liquid-operated booster, particularly at the time the liquid-operated booster is abruptly actuated (for example, at the time the brake pedal is abruptly actuated). Incidentally, it is considered that this foreign sound is generated by the pressurized liquid which flows, all on a sudden, into a servo chamber of the liquid-operated booster causing the flow rate at the valve portion to become extremely fast. This foreign sound, different from the above-mentioned large vibrations and noises, seems to draw the driver's keen attention. That is, this foreign sound is a big problem for the driver, etc. and needs to be solved separately from the noises and vibrations due to the pressurized liquid. This foreign sound occurs not only in the reciprocal type pump but also in the radial piston type pump which generates less vibrations and noises.